Steel is widely used in the construction, transport and energy industries due to its high strength and relatively low production cost. Steel structures, particularly those made of high temperature and/or carbon steels, are usually painted with protective coatings to prevent them from corrosion. Before painting, the steel substrates need to be properly cleaned, e.g., by grit blasting or power tool cleaning, so that the loosely bonded mill scale and rust are removed to ensure good paint adhesion. A problem that has long been plaguing the painting operations on large steel structures is the formation of “flash rust” on cleaned steel surfaces, i.e., red rust reforms after the surface is cleaned, especially in humid environments. For this reason, paint suppliers usually specify a very short time window for painting after the surface preparation step (e.g., within 8 hours). This is not desirable from an efficiency and flexibility point of view, particularly when a large surface area needs to be painted.
There have been various pretreatment coatings that can minimize the formation of flash rust. Chromate and phosphate conversion coatings are widely used on steels to improve the corrosion resistance. These coatings usually require rinsing of the metal substrate to remove applied pretreatment solution. Moreover, hexavalent chromium compounds used in these systems are classified as carcinogenic. For these reasons, chromate and phosphate conversion coatings are undesirable for large-area industrial applications.
Much effort has been made in developing compositions and methods for producing chromium-free pretreatments. U.S. Pat. No. 7,507,480 describes metal surfaces having thereon an ultrathin (e.g., less than ten nanometer thickness) corrosion-resistant film, which is said to render the metal surface corrosion-resistant. The corrosion-resistant film includes an amido-functionalized silanol component in combination with rare-earth metal oxide nanoparticles. The corrosion-resistant film according to this patent is preferably dried at elevated temperatures.
U.S. Pat. No. 5,108,793 describes forming a silica coating by rinsing the steel with an alkaline solution containing dissolved silicate and metal salt, at an elevated temperature preferably at least 45° C. The steel is dried to form a silica coating having a thickness of at least 2 nm, Thereafter, the silica coated steel is rinsed with an aqueous solution containing 0.5-5 Vol. % organofunctional silane. If the sheet is painted, the silane film forms an adherent bond between the paint and silicate coating. The silane forms a relatively adherent covalent bond between the silicate coating and an outer paint layer. However, the method described in this patent involves two separate rinsing steps and a drying step in between.
U.S. Pat. No. 5,322,713 describes metal sheet protected against corrosion by a silane treated inorganic aluminate coating. A thin aluminate coating was formed by immersing a galvanized steel sheet into an alkaline solution containing 0.005M dissolved aluminate for about 30 seconds. The sheet was dried to form an adherent aluminate coating having a thickness of at least 5 nm. The aluminate coated sheet was immersed into a solution containing preferably 1.0 vol. % hydrolyzed organofunctional silane for about 5 seconds forming a silane film having a thickness of at least 2 nm on the outer surface of the aluminate coating. Thereafter, the silane treated aluminate coated sheet was painted. The silane film formed a covalent bond between the outer paint layer and the inner aluminate layer. A steel sheet treated with the silane sealed aluminate coating was said to have good corrosion protection, good paint chipping resistance and good paint formability. Again, the method described in this patent involves two separate rinsing steps and a drying step in between.
U.S. Pat. No. 5,433,976 describes painted metal sheet pretreated with an insoluble, composite layer containing siloxane. The composite layer is formed by rinsing the sheet with an alkaline solution containing at least 0.005M of a dissolved silicate or a dissolved aluminate, preferably at least 0.8 vol. % of an organofunctional silane and at least 0.2 vol. % of a crosslinking-agent having two or more trialkoxysilyl groups. After the sheet is dried, the composite layer has a thickness of at least 1 nm. After being painted, the siloxane is said to form a tenacious covalent bond between the paint and the metal substrate.
As evidenced by the effort of previous workers, good paint adhesion is always a sought-after feature for a paint system. Poor adhesion is related to common coating degradation mechanisms such as peeling, flaking, blistering, corrosion creep and cathodic disbonding. Among those, cathodic disbonding is an important degradation mechanism for organic coatings on steel structures subjected to coating damage that exposes bare metal to the environment. The mechanism of cathodic disbonding is not understood in every detail. Researchers have generally agreed that the cathodic reactions due to the corrosion of the exposed metal leads to the formation of an alkaline water film under the coating that causes the disbonding. Studies have shown that cathodic disbonding represents the most severe form of adhesive bond degradation. It is known that coatings with good cathodic disbondment resistance tend to have long service life in the field. For this reason, the cathodic disbondment resistance is considered as a key property for paint systems that are exposed to aggressive outdoor environments. However, this property was largely neglected for prior steel pretreatment methods.
In view of the foregoing, one of the objectives of the present invention is to provide an easy, simple and environmentally friendly pretreatment solution and method for steels, which method can temporarily passivate the cleaned steel surfaces and inhibit the formation of flash rust in humid environments, thereby extending the time window for painting operation. The present method does not use toxic materials, does not include steps at elevated temperatures and does not involve complex multiple-step processing.
Another objective of the present invention is to provide a steel pretreatment solution and method that can enhance the adhesion and cathodic disbondment resistance of the overlying paint systems, therefore extending the service life of the paint systems.